There are a number of rotary cutting tools used to create, extend, enlarge or do other work within subterranean boreholes, which may be boreholes drilled in the course of oil and gas exploration and production. Drill bits are one instance of such tools. Others include reamers which are used to maintain or enlarge the diameter of a borehole and mills which are used to remove material which has been placed in a borehole. Such tools commonly have a support structure for cutting elements and separate cutters of hard material secured to the support structure. In some tools, the cutters are formed of hard material such as tungsten carbide or a mix of tungsten carbide and other material(s). In some tools, the cutters comprise a compact of polycrystalline diamond which may be supported on a body of other hard material such as tungsten carbide. Such cutters with polycrystalline diamond are commonly referred to as PDC cutters. When a tool has separate cutters of hard material (with or without polycrystalline diamond), the cutters are generally fabricated separately and subsequently attached to the support structure. This may be done by brazing.
During use of a cutting tool, its cutters undergo wear, which may be wear by abrasion, although chipping and breakage can also occur. Tripping a worn tool out of a borehole is time-consuming and therefore expensive. Tripping a tool out of a borehole before the amount of wear makes it necessary to do so is therefore a significant waste of resources. There are schemes for estimating wear of a drill bit from surface or downhole parameters such as rate of penetration, torque, rotary speed and weight on the tool. One such scheme for predicting wear comes from work of Detournay et al in “Drilling response of drag bits: theory and experiment” International Journal of Rock Mechanics and Mining Sciences vol 45 pp 1347-1360 (2008) and another from Rashidi et al in “Real-Time Drill Bit Wear Prediction by Combining Rock Energy and Drilling Strength Concepts” Society of Petroleum Engineers paper SPE 117109.
Cutting tools such as drill bits may incorporate sensors of various types. The information collected from such sensors whilst the drill bit is in use may be stored in electronic memory accommodated within the cutting tool itself and/or may be transmitted to the surface. U.S. Pat. No. 7,168,506 shows a drill bit which is provided with a number of sensors. Several kinds of sensors are mentioned in this document including wear sensors. U.S. Pat. No. 8,006,781 discloses a drill bit in which sensors intended to detect wear may be constructed to carry an electrical signal current whilst intact and to be destroyed by wear, so that the wear can be revealed by the circuit ceasing to carry the signal current. In U.S. Pat. No. 8,006,781, the wiring to detect wear extends within the body of the drill bit beneath the hard cutters.